In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added gaseous products from lower-fuel-value carbonaceous feedstocks, such as biomass, coal and petroleum coke, is receiving renewed attention. The catalytic gasification of such materials to produce methane and other value-added gases is disclosed, for example, in U.S. Pat. No. 3,828,474, U.S. Pat. No. 3,998,607, U.S. Pat. No. 4,057,512, U.S. Pat. No. 4,092,125, U.S. Pat. No. 4,094,650, U.S. Pat. No. 4,204,843, U.S. Pat. No. 4,468,231, U.S. Pat. No. 4,500,323, U.S. Pat. No. 4,541,841, U.S. Pat. No. 4,551,155, U.S. Pat. No. 4,558,027, U.S. Pat. No. 4,606,105, U.S. Pat. No. 4,617,027, U.S. Pat. No. 4,609,456, U.S. Pat. No. 5,017,282, U.S. Pat. No. 5,055,181, U.S. Pat. No. 6,187,465, U.S. Pat. No. 6,790,430, U.S. Pat. No. 6,894,183, U.S. Pat. No. 6,955,695, US2003/0167961A1, US2006/0265953A1, US2007/000177A1, US2007/083072A1, US2007/0277437A1 and GB1599932.
Reaction of lower-fuel-value carbonaceous feedstocks under conditions described in the above references typically yields a crude product gas and a char. The crude product gas typically comprises an amount of particles, which are removed from the gas stream to produce a gas effluent. This gas effluent typically contains a mixture of gases, including, but not limited to, methane, carbon dioxide, hydrogen, carbon monoxide, hydrogen sulfide, ammonia, unreacted steam, entrained fines, and other contaminants such as COS. Through processes known in the art, the gas effluent can be treated to remove carbon dioxide, hydrogen sulfide, steam, entrained fines, COS, and other contaminants, yielding a cleaned gas stream comprising methane, carbon monoxide, and hydrogen. Carbon monoxide may optionally be removed or converted at some point prior to hydrogen separation, yielding a cleaned gas stream comprising methane and hydrogen.
For some applications, it may be desirable to recover a gas stream that is enriched in methane. In some situations, it may even be desirable to recover a gas stream that almost entirely comprises methane. In such situations, the cleaned gas stream must undergo additional processing to remove substantially all of the hydrogen and, if present, carbon monoxide. On the other hand, hydrogen and mixtures of hydrogen and carbon monoxide (“syngas”) have utility as an energy or raw material source.
In some situations, such as in the context of the present invention, it may be desirable to collect hydrogen and, optionally, carbon monoxide and recycle them as a raw material or energy source for a subsequent catalytic gasification process. Thus, there is a continued need for processes which permit the efficient recovery of methane, and also permit recovery of separate gas streams of carbon monoxide and hydrogen.